1. Field of the Invention
The present invention relates to an amplifier having an improved push-pull output stage and, in particular, to an amplifier suitable for an IC fabrication.
2. Description of the Related Art
FIG. 1 shows an ordinary voltage follower circuit. Here, reference numerals 11 and 12 represent a differential input stage and output stage, respectively. In the circuit shown in FIG. 1, the output stage 12 is of an emitter follower configuration and a sink current at the output stage OUT is determined by an internal constant current source 13. As a way for increasing an output sink current, an output stage 21 is provided as a push-pull output circuit as shown in FIG. 2. In the circuit shown in FIG. 2, a transistor Q4 is driven from a differential input stage transistor Q1 side via a current mirror circuit 22 and transistor Q21. A transistor Q3 is driven from a differential input stage transistor Q2 side via current mirror circuits 23 and 24 and transistors Q22. With the collector current and base current of the transistor Q3 represented by IC and IB, the following relation holds: EQU I.sub.C /I.sub.B =h.sub.FE (current amplification factor)
If an input IN2 is to be connected to the collector of the transistor Q2 by omitting the current mirror circuits 23 and 24 which provide a cause for a complicated arrangement, then an equal potential is applied to the collector and emitter of the transistor Q2, that is, a potential on the output OUT becomes a level dropped by a base to-emitter voltage V.sub.BE, causing the transistor Q2 to be driven into saturation. This connection cannot be achieved with an omission of the current mirror circuits 23 and 24, resulting in a complex circuit.
FIG. 3 shows a known output stage 31 using transistors Q11 and Q12 and constant current sources 32 and 33 which eliminate the necessity of paying particular attention to the levels on INl and IN2 in FIG. 3. This circuit requires two constant current sources 32 and 33.